1. Field of the Technology
The present technology relates to a charging apparatus and an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus (hereinafter simply referred to as “image forming apparatus”) has been widely spread in the form of, for example, copiers, printers, and facsimile apparatus because they allow images of high quality to be formed through simple operations in a short time and because they can be easily maintained and managed. For example, an electrophotographic image forming apparatus includes a photoreceptor drum, a charging apparatus, an exposure section, a developing section, a transfer section, and a fixing section. The photoreceptor drum is a member in the form of a roller having a photosensitive layer on a surface thereof. A charging apparatus charges the surface of the photoreceptor drum at a predetermined electric potential having a predetermined polarity. The exposure section forms an electrostatic latent image on the surface of the photoreceptor drum thus charged. The developing section supplies toner to the electrostatic latent image on the surface of the photoreceptor drum to form a toner image. The transfer section transfers the toner image on the surface of the photoreceptor drum onto a recording medium. The fixing section fixes the toner image on the recording medium. Thus, an image is formed on the recording medium.
A corona charging apparatus including a corona electrode, a grid electrode, shield case, and a support member is primarily used as the charging apparatus included in the image forming apparatus. The corona electrode undergoes corona discharge to charge the surface of the photoreceptor drum at a predetermined electric potential having a predetermined polarity. The grid electrode is provided between the photoreceptor drum and the corona electrode, it adjusts the amount of the charge imparted from the corona electrode to the surface of the photoreceptor drum to control the potential at which the photoreceptor drum surface is charged. The shield case is provided to enclose the corona electrode excluding the space between the corona electrode and the photoreceptor drum. The support member supports the corona electrode and the grid electrode. The corona charging apparatus can control the potential charged on the photoreceptor drum surface substantially at an exact value. For example, a metal plate electrode (hereinafter also referred to as “saw-tooth electrode”) having a plurality of needle-like portions (pointed projections) is used as the corona electrode of the corona discharge apparatus. A saw-tooth electrode is more advantageous than a wire electrode or the like in that it has a smaller number of components, has a longer life, generates a smaller amount of ozone, and has less failures such as breakage. A saw-tooth electrode is manufactured by etching a metal plate which is primarily made of an iron-type metal material such as stainless steel or nickel to form a plurality of needle-like portions on the plate. Such a saw-tooth electrode made of an iron-type metal material has a problem in that it is vulnerable to oxidation attributable to ozone generated as a result of corona discharge of the corona electrode, although it has high durability. When a saw-tooth electrode is used for a long time, it may be unavoidable to use the electrode in a highly humid environment (an environment including a relatively great amount of moisture) and to put the electrode in contact with ozone. Moisture and ozone in the air result in corrosion such as rusting, and durability of the electrode is reduced by nitrogen oxides thus deposited on the surface of the same. In addition, the corona discharge from the needle-like portions reduces the controllability of a voltage applied to the saw-tooth electrode. As a result, the potential charged on the surface of the photoreceptor drum becomes uneven. Therefore, it is not possible to always impart a desired charging potential to the surface of the photoreceptor drum stably, which constitutes a problem to be solved.
In consideration to such a problem, proposals have been made to introduce corona charging apparatus including a saw-tooth electrode in which a layer of an ozone-dissolving catalyst is provided on a surface of the saw-tooth electrode excluding the tops (tips) of needle-like portions of the saw-tooth electrode and the neighborhood of the tips (see Japanese Unexamined Patent Publication JP-A 10-090974 (1998) for example). As described in the paragraph [0035] of the publication, the ozone-dissolving catalyst may be a metal oxide or coconut shell-based activated carbon. According to the technique disclosed in JP-A 10-090974, any reduction in the durability of a saw-tooth electrode attributable to ozone is prevented by providing an ozone-dissolving catalyst layer on the saw-tooth electrode excluding the region where corona discharge takes place. However, since the saw-tooth electrode undergoes corona discharge in response to the application of a high voltage, the ozone-dissolving catalyst layer itself is damaged as time passes under the influence of corona discharge, and the ozone-dissolving capability of the layer is reduced. Therefore, according to the technique disclosed in JP-A 10-090974, it is not possible to prevent reduction in the durability of a saw-tooth electrode attributable to ozone throughout the life of an image forming apparatus.
A saw-tooth electrode provided by coating a surface of a nickel plate with a gold plating layer or platinum plating layer is described in paragraph [0041] of JP-A 10-090974. The gold plating layer or platinum plating layer is advantageous in suppressing the generation of ozone and preventing the corrosion of nickel. However, since they are expensive metals, how to control the thickness of films of such metals is a significant problem. From the viewpoint of durability, the effect of preventing ozone generation, and the effect of protecting nickel, it is normally required to form a gold plating layer with a thickness of 0.3 μm or more. However, when the thickness of a gold plating layer is too great, there will be a significant increase in the manufacturing cost of the same. It is therefore required to control the thickness of a gold plating layer accurately such that it will stay in a very narrow range. For this purpose, plating conditions must be strictly controlled, which unavoidably increases the complicatedness of plating operations and results in a significant increase in manufacturing cost.
Proposals have been made to introduce charging apparatus including a saw-tooth electrode and a grid electrode in which a nickel plating layer containing polytetrafluoroethylene particles (hereinafter referred to as “PTFE particles”) is provided on at least one side of a needle-like electrode (the nickel plating layer will be hereinafter referred to as “PTFE-containing nickel plating layer”) (see Japanese Unexamined Patent Publication JP-A 2006-201488 for example). The PTFE-containing nickel plating layer can prevent the generation of ozone in the vicinity of the saw-tooth electrode to protect the saw-tooth electrode from ozone and moisture in the air, which allows the life of the saw-tooth electrode to be extended. Further, since the PTFE-containing nickel plating layer itself has high durability, the layer will be subjected to substantially no degrading in its function of protecting the saw-tooth electrode even when exposed to corona discharge. However, the saw-tooth electrode protecting function of the PTFE-containing nickel plating layer may be degraded unless PTFE particles are uniformly dispersed in the nickel layer. In order to disperse PTFE particles in the nickel layer uniformly, strict process management must be conducted at the plating process. Therefore, an increase in manufacturing cost is inevitable also when a PTFE-containing nickel plating layer is provided, although the increase is not as great as that in the case of a gold plating layer. Therefore, there are demands for a technique which allows PTFE particles to be reliably and uniformly dispersed in a nickel layer without increasing manufacturing cost or a technique which keeps the saw-tooth electrode protecting function of a nickel layer substantially unaffected even when PTFE particles are not dispersed in the layer uniformly.